I've asked this question of literally dozens of contractors over the years and frankly have never gotten a straight answer. While they all have a preference as to what they use, none of them have EVER been able to give me a concrete answer on what specifically. In fact, you've probably noticed that on many of them the labels are identical.

Funny thing: There's a similar mystery between "Topping" and "Joint Compound". I know contractors who use one of the two exclusively for all projects... so apparently there's not that much of a difference.

Now, that having all been said, I'll be very curious to see if someone in the know has a concrete answer...

Light weight compound sands easier and goes on smoother with less air bubbles.
Its easier to feather the edge of a joint with.
Regular joint compound is good to use for the first 2 application to a joint. The final application where you need to make the joint smooth and prevent build along the edges light weight works better for this.
Light weight works good if you need to skim coat a whole wall because as mentioned it goes on smooth and is easy to spread.

As for topping compound, this is usually used to set tape joints. It is very difficult to sand. Some guys will use if for skimming a wall where they put it on smooth enough where they don't have to sand the wall.

If I have a large drywall job I will buy light wieght by the bucket just because it's a personal preference. If it a smaller job I'll buy powder form (sheetrock brand) 45 or 90 and mix my own spackle..

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My brother and I plan on putting down some laminate flooring but the existing sub floor very uneven. We are hoping to get out of replacing as much as we can and use some of that leveling material.
So my question is how much is the max suggested? Is it 1/4" or ?
Thanks for the help.